Phasic 3s are the only ones that are really worth anything. The phasic 2s do the exact same damage to shields and barriers per hit as Disruptor IV's (10x multiplier of 10% vs 4x multiplier of 25%) but lack all of the other advantages (tech priming, stuns, greater health damage, trolling biotics etc).
I actually use Disruptor IV's on the vanilla Widow and Javelin a lot (mostly when using them on the QFI, for the powered up tech bursts). They still can OHK most shielded targets, set up properly. BW and Mantis generally need Phasics to do the same, though. One can also get the Valiant to the point where it can OHK shielded targets (sort of) on headshots with infiltrator cloak damage bonus and phasics 3 on certain setups.